1. Field of the Invention
The present invention relates to a method and apparatus for adjusting ink drop velocity, and, more particularly, in one embodiment, to a method and apparatus for adjusting ink drop velocity irrespective of sensors.
2. Description of the Related Art
An ink jet printer typically includes a printhead, which is carried by a carrier. The printhead is fluidly coupled to an ink supply. Such a printhead includes a plurality of nozzles having corresponding ink ejection actuators, such as heater elements.
Ink is jetted from the nozzles onto a print medium at selected ink dot locations within an image area. The carrier moves the printhead across the print medium in a scan direction while the ink dots are jetted onto selected pixel locations within a given raster line. Between passes of the printhead, the print medium is advanced a predetermined distance and the printhead is again moved across the print medium.
Ink jet printers may utilize a single printhead, or multiple printheads. For example, some ink jet printing systems utilize a monochrome ink cartridge including a monochrome, e.g., black, printhead, and a color ink cartridge including a color printhead having cyan, magenta and yellow nozzle groups. In another type of ink jet printing system, each printhead is connected to a respective remote ink supply.
The manufacture of printheads involves certain manufacturing tolerances that result in manufacturing variations (e.g., variations in sheet resistance of the material used in the heater elements; mask alignment variations, which lead to variations in the width and length of heater elements; the rise and fall times of transistors that drive the heater elements; the thickness of the layer between the heater element and the ink, which influences heat transfer to the ink; the ink chemistry; and the voltage level of the power source), which in turn result in printheads that require differing amounts of energy to attain a drop velocity deemed suitable (e.g., high enough) for attaining a desired print quality. Thus, typically, from printhead to printhead, the amount of energy required to attain a suitable drop velocity varies.
Because of these manufacturing variations, an energy level for driving such printheads will be selected so that most printheads will attain a certain minimum drop velocity (e.g., 400-600 inches per second). This energy level is a statistical average value meant to encompass the largest range of printhead variations possible. Because the same predetermined amount of energy is used for each printhead, the energy is not optimized for a particular printhead.
One problem with this manner of ink delivery is that variations in printheads lead to inefficiencies in printhead operation. The result is ink drop velocity variations and difficulty in maintaining nominal head temperatures. Another problem is that driving ink jet heater elements at an energy level required to jet ink at an acceptable drop velocity means overdriving some printheads. By overdriving printheads, the overdriven nozzles can fail prematurely due to electromigration of the heater element.
What is needed in the art is a method and apparatus that reduces variations in drop velocities among a type of printhead, and/or provides for fire energy adjustment for the printhead.